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"Relative humidity" is a function of air pressure, temperature, and humidity. To simplify my question, I'll leave air pressure out of it. So, here goes: I have a humidifier that keeps the "relative humidity" of the piano room at 42%, even when it gets as cold as 55 fahr. The relative humidity stays at 42% when it's as warm as 80, too.

Question: is the temperature change ITSELF a problem, even when the relatively humidity is kept at 42%? Are there parts of the piano that shrink and expand relative to temperature changes, even when relative humidity is kept constant??? Is it "critical" in any way that the temperature of a piano stay within a certain range, even when relative humidity is constant?

The humidifier has of course a read out, but I would never trust that alone. I have 3 other RH monitors in the room. All show the room is at a constant 42 per cent relative humidity, regardless of the temperature (because that's what a good humidifier is supposed to do... maintain a constant RH).

The piano stays very, very in tune, BTW. But I've often wondered how TEMPERATURE, as an "independent variable" (as our high school science teacher used to call it), affects pianos (if at all!) The RH might remain constant, but the temperature of the room can of course change quite a bit.

As I understand it, the important thing is to maintain a constant moisture level in the various wooden parts of the piano. The moisture content in the wood will depend upon the RH in the environment, hence why it is important to keep the RH constant (or at least within a limited range). I believe that the temperature is not relevant; if this is wrong, I hope that somebody will explain why the temperature should matter if the RH is constant.

Temperature can effect a piano. Metal expands and contracts with temperature changes, strings are made of metal. I have seen strings go flat that had sunlight hitting them from the window next to the piano.

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Stewart MoorePiano Technician North Central and North East Kansas

Relative Humidity is just that - Relative. Warm air can hold more moisture than cold air. If the temperature in the room holding the piano lowers, so does the amount of moisture in the air if the RH remains the same. The only way to keep the actual amount of moisture in the air constant is to increase the RH as the temperature drops. This only works up to a point. The real question is - Does the piano prefer a constant "specific moisture level" or is it OK with a constant RH? In my experience, a small change in temperature 18C plus or minus 3C and a RH of 47 plus or minus 5 holds a piano tuning very nicely.

Relative Humidity is just that - Relative. Warm air can hold more moisture than cold air. If the temperature in the room holding the piano lowers, so does the amount of moisture in the air if the RH remains the same. The only way to keep the actual amount of moisture in the air constant is to increase the RH as the temperature drops. This only works up to a point. The real question is - Does the piano prefer a constant "specific moisture level" or is it OK with a constant RH? In my experience, a small change in temperature 18C plus or minus 3C and a RH of 47 plus or minus 5 holds a piano tuning very nicely.

I guess I'm really missing something here. The whole point of maintaining a constant RH is so that the room (and everything in it) maintains a more or less fixed moisture level, at VARYING temperatures.

Ergo, 50 degrees or 80 degrees, no problem: a humidifier (or dehumidifier, if necessary) maintains a constant level of moisture.

Relative Humidity is a measure of the ratio of the amount of moisture in the air relative to what it is capable of holding. The term "dewpoint" is the temperature at which the air contains 100% of the moisture that it is capable of holding, and, if the the actual temperature drops to the dewpoint temperature, you get fog.

At very low temperatures, such as are found in the arctic, there is very little moisture in the air, even when the RH is 100%. At very high temperatures, such as are found near the equator, there is a lot of moisture in the air, even at a low RH.

You may have noticed that when the temperature drops in the room, such as at nighttime, the humidifier stops running for a while. That occurs because the RH is rising as the temperature drops. After a while however, the air starts to dry out, the RH drops below the set-point, and the humidifier starts again. There will always be less moisture in the air for a constant RH at a lower temp.

OK... so even where the RH is kept at a constant, say, "42 per cent" that's not enough.

To the contrary. Not only is it quite enough, it is perfect.

Originally Posted By: johnlewisgrant

A more or less constant temperature, say, 70 degrees also necessary, even when RH is constant.

Not for the sake of the wood. Only for the sake of preventing expansion or contraction of the metal. But within the typical temperature range in a home, I would regard those changes in the metal as negligible.

Originally Posted By: johnlewisgrant

That's because a change in temperature WILL change the humidity of the piano, EVEN WHEN THE RH IS KEPT AT A CONSTANT NUMBER?

No. If rH is constant, then the moisture content of the wood will also remain constant - irrespective of the temperature. So, a constant temperature is not required to keep the piano's wood at constant moisture content. The moisture content of the wood is only dependent on the relative humidity of the surroundings. It is NOT dependent on the temperature. The environment in which you keep your piano is very friendly, and more than sufficient. In fact, it's pretty much ideal.

Again: a change in temperature will NOT change the moisture content of the piano, provided you keep the relative humidity constant (which you are!) So there's no reason to fuss any further.

Also, please disregard postings about "warm air can >hold< more moisture than cold air". This is a layman's notion that is all over the internet, and even in some textbooks. The concept of relative humidity has nothing to do with the "capacity" of the air. It is just as applicable to a vacuum or other gases. Relative humidity is the amount of water vapor present in a given space, expressed as a percentage of the maximum amount of water vapor that can be present in this space, before condensation will take place (that maximum point is called dew point or saturation point). You can see that neither "air" nor its "holding capacity" comes into this definition at all.

Go in peace and enjoy your piano. You're providing it with a very good life. (As evidenced by the super-stable tuning.)

Mark I disagree, if the RH level is kept constant but the temperature change, there is a little change in the wood moisture content level.

This is actually helping us as the pianos may accept lower levels of RH during winter, assuming the temperature inside is not kept too high (not excessive heating)

the wood moisture content tables are saying what is the wood humidity content and what dimensional change may occur.That said, practically , having a 42% level will certainly be good enough (even if the "factory environment" state 42% FOR 21°c - 69.8 F

(the 2 numbers linked are giving a good appreciation of the wood dryness)

I dont know if on a all year long period, 42% is better than 45 % for instance, I would suggest that 42% in summer, with very high temperatures, is a somewhat dry environment, while 42% during the cold season is even something difficult to obtain in most places, so I would keep those numbers, possibly trying to obtain more than 42% in winter mean sending a lot of water in the air.

I stay persuaded that that piano is in very stable conditions , as it is (particularly when comparing with the situation of most pianos

You can see your wood humidity can range from 7.9% to 8.2 % depending of the temperature. This is very small, but remind that the wood saturation is at 28% moisture (in weight) +- (to be considered 100%)Dry wood (to be used for pianos) is 7% to 10% (those days, while it was 9% to 14% before modern heating came in the houses, so older pianos need more than 42% - 21c° (69.8°F) in theory.

typical variations of the wood moisture content during the year are ranging from 4 to 14% outside.

The wood dimensional change is really very small, from 7.9* to 8.2 (a 1000 dimension turn to 1000.1 or 1000.2) so in the facts the process exists, but I understand why air RH is used as a final measure , hoping it can reassure you

Edited by Olek (03/04/1304:29 AM)

_________________________
Professional of the profession.

I wish to add some kind and sensitive phrase but nothing comes to mind.!

Technically, I stand corrected. It appears from the EMC calculator that at a constant r.h., temperature does have a SMALL influence on the EMC of the wood.

E.g. at a constant 42% r.h.:At 50°F the EMC is 8.2%, while at 90°F it is only 7.9%. That is 0.3% difference.

However, please compare this to fluctuations in r.h. at a constant temperature, e.g. a constant 65°F:At 40% r.h. the EMC is 7.8%, but at 50% r.h. it is already 9.3% and at 60% it is a whopping 11.1% !!!

One can see that the influence of r.h. is much, much larger than that of temperature. I would call the influence of temperature practically negligible.

So, for all practical purposes, given the typical temperature fluctuations in a home, I stand by my earlier post: if you are already controlling r.h., that is quite enough. It is not necessary to keep temperature perfectly constant.

This is actually helping us as the pianos may accept lower levels of RH during winter, assuming the temperature inside is not kept too high (not excessive heating)

OK, let's get some real numbers from the website you linked.

Scenario 1: summer, temperature is 75°F (24°C) and r.h. is 45%.The EMC of the wood is 8.4%.

Now, you say in winter we can "accept lower r.h.". So let's do the sums.

Scenario 2: winter, temperature is 50°F (10°C). Inside a home, that's really cold.How far can we allow the r.h. to drop, while keeping 8.4% EMC?Answer: to 43-44%. That's just ONE TO TWO percent, Isaac!If we "allow" 35% in winter, as you would perhaps suggest, then the EMC will drop to 7.1% !!

So sorry, your argument doesn't hold any water (pun intended). Lower r.h. levels are not somehow more acceptable in winter than they are in summer.

[quote=Mark R.][quote=johnlewisgrant]Also, please disregard postings about "warm air can >hold< more moisture than cold air". This is a layman's notion that is all over the internet, and even in some textbooks. The concept of relative humidity has nothing to do with the "capacity" of the air. It is just as applicable to a vacuum or other gases. Relative humidity is the amount of water vapor present in a given space, expressed as a percentage of the maximum amount of water vapor that can be present in this space, before condensation will take place (that maximum point is called dew point or saturation point). You can see that neither "air" nor its "holding capacity" comes into this definition at all.

I have to take issue with the above statements. While it is true that air cannot "hold" water vapor, it is true that the maximum amount of water vapor that is a constituent of air in a given space varies with temperature. Check any reputable source, the NOAA for example, and you will find that air at 40C can have up to 51.1gm/m3 water vapor as a constituent; at 20C up to 17.3gm/m3, and at 0C it can have up to 4.85gm/m3. In each case the RH would be 100%. Therefore, at 42%RH, there would be 21.5, 7.27, and 2.04gm/m3 of water vapor at 40, 20 and 0C respectively as a constituent of the air.

I think, though, that Mark R. is on the right track. If the moisture content of wood remains relatively constant due to the constant vapor pressure of a constant RH, then the piano should be stable.

I have to take issue with the above statements. While it is true that air cannot "hold" water vapor, it is true that the maximum amount of water vapor that is a constituent of air in a given space varies with temperature. Check any reputable source, the NOAA for example, and you will find that air at 40C can have up to 51.1gm/m3 water vapor as a constituent; at 20C up to 17.3gm/m3, and at 0C it can have up to 4.85gm/m3. In each case the RH would be 100%. Therefore, at 42%RH, there would be 21.5, 7.27, and 2.04gm/m3 of water vapor at 40, 20 and 0C respectively as a constituent of the air.

Sorry, but no.

Correctly, the statement should read (possibly with very slight changes to the actual numbers):

Quote:

The maximum amount of water vapor that is a constituent of air in a given space varies with temperature. Check any reputable source, the NOAA for example, and you will find that aira space at 40C can have up to 51.1gm/m3 water vapor as a constituent; at 20C up to 17.3gm/m3, and at 0C it can have up to 4.85gm/m3. In each case the RH would be 100%. Therefore, at 42%RH, there would be 21.5, 7.27, and 2.04gm/m3 of water vapor at 40, 20 and 0C respectively as a constituent of the airspace.

Catch my drift? Your line of thought is absolutely fine, except that it doesn't matter whether the water vapor co-occupies a space with air, or a cow's fart, or vacuum. The concept of relative humidity is only dependent upon the water vapor pressure and the temperature in that volume of space. The air (or cow's fart) are only spectators.

The point was, and still is, Marty, that no "carrier gas" is required for r.h. concepts to hold true.

Water can evaporate or sublime into a vacuum, i.e. be the only gas present in that space. Obviously, once there is water vapor in that space, it's not a vacuum anymore. Pray don't be facetious. (I know, that's a bit much to ask. )

Originally Posted By: Mwm

However, this is a forum for earth based life forms

And here I thought this is a forum for pianos...

Edited by Mark R. (03/05/1302:38 AM)Edit Reason: added response to Mwm here, rather than in a separate post.

A forum is an assembly for the discussion of public issues, presumably by "the public", which, in common usage, implies a limited group of earth based life forms, specifically, homo sapiens, and, in this case, the discussion is about pianos. Being a person who does not believe in absolute truth, only observed reality, if I saw a group of vacuum packed pianos discussing issues regarding their tuners or players, then I would accept the possibility of a forum for pianos.

You are correct. But the majority of the technical concepts being discussed in this forum are rife with inaccurate or wrong terms, and yet, with some time and patience, these people are happily (mostly) solving issues that they face daily.

Another PW forum, such as Physics World, might be more appropriate for our intercourse.

But the majority of the technical concepts being discussed in this forum are rife with inaccurate or wrong terms, and yet, with some time and patience, these people are happily (mostly) solving issues that they face daily.

Sure about this? I can’t count the times over forty years I have attended an instrument that has been “repaired” only to have to “solve issues” and charge the pseudo repair person some more.